The Excretory System

Chapter 44

What you need to know!

• Different waste products, which animal groups produce each, and why.

• The components of a nephron, and what occurs in each region.

• How hormones affect water balance by acting on the nephron.

The Excretory System

• Made of kidney, ureter, urinary bladder, and urethra.

• The kidney has 2 functions:

1. Kidneys are a filtering/dialysis system that removes water soluble waste (urea NH4+), toxins, and drug fragments from the blood

2. Kidneys are also part of a system involved in the homeostasis of blood: pH, water, salt concentration (tonicity - blood needs to be isomolar to all body cells)

The Excretory System

• Smallest unit: nephron• 1 million nephrons• Body contains about 5L of blood, 1,500L of

blood pass through the kidney per day• 180L of filtrate are produced and reduced

to about 1.5L of urine/day

Anatomy of a Kidney

The Kidney is supplied by the renal artery that branches off of the aorta and becomes the renal vein after the capillary bed. Outside tissue of the kidney is called the cortex, followed by the medulla, the inside of the kidney is called the kidney pelvis.

Anatomy of a Nephron

Production of Urine – 4 steps

1. Filtration

2. Reabsorption

3. Secretion

4. Excretion

1) Filtration

Blood pressure forces blood through capillary balls called glomerulus. Small molecules from the blood are forced through the capillary walls and are collected in the Bowman’s capsule. Molecules include:

• NaCl• HCO3-• H+• Urea• Glucose• Amino Acids• (Some drugs)

Larger blood particles like cells, platelets, and proteins cannot pass through. The remaining blood is highly concentrated but nutrient poor.

2) Reabsorption

Bowman’s capsule releases diluted urine into the proximal tubule. Reabsorption of some molecules back into the blood stream takes place in the proximal tubule, the Loop of Henle (water), and the distal tubule.

Molecules reabsorbed include:• Nutrients: glucose, AA• NaCl (Sodium Chloride = salt)

• HCO3- (Hydrogen Carbonate = ion)

• Water follows the active transport of salt molecules back into the blood stream

3) Secretion

• In the proximal and distal tubules some ions like K+ and H+ are actively secreted from the blood back into the nephron for pH/ion homeostasis.

• Also some byproducts from drugs/medications are secreted here.

4) Excretion

• Concentrated urine includes: wastes, some water and some salt. All flow into the ureter and bladder (impermeable walls) and are excreted.

Composition of Urine

• Ammonia (NH3) from breaking down proteins (amino group) during digestion, and destruction of old cells.

• Ammonia is toxic and is converted into Urea (NH4+ / a salt)

• Kidneys regulate tonicity by excreting/secreting sodium Na+, and K+ levels. pH is regulated by excreting H+

• Urine leaving the bladder in a healthy person is sterile: free of pathogens

• Urine can either be hypotonic, isotonic, or hypertonic compared to body fluids depending on hydration status

Composition of Urine

• Glucose is usually completely reabsorbed• Diabetes: blood-sugar is too high (lack of

insulin D1, or glucose absorption into cells D2)– Kidneys try to excrete excess sugar in the urine

as part of homeostasis

• Bacterial infections: Glomerulus walls are compromised by bacteria infection the kidney (they get there from the bladder). The filter breaks down. Blood cells leak into the Bowman’s capsule and are excreted in the urine (brown urine color)

Composition of Urine

• Kidney Stones: The most common kidney stones are composed of calcium, which can form stones inside the urinary tract. Usually they are so small you don’t even notice them, but if they get large enough they can block the flow of urine, causing extreme pain as well as blood in the urine. This is mostly a hereditary disorder. It is most common in white men.

Hormonal regulation of urine production

• The control center in the brain measures tonicity and interacts with the kidneys via ADH (Anti-diuretic hormone). ADH is a water-loss limiting hormone.

• Diuresis means increased urination• Dehydration: ADH increases = less

frequent urination• Coffee and Alcohol: block ADH =

increased urination (increased dehydration)– Hangovers are often due to dehydration

Homeostasis/Urine Production in Other Organisms

• Protists have a contractile vacuole (tiny organelle pump) that collects water out of the cytoplasm

• Insects have Malphigian tubules next to their digestive system which collect fluid from the hemolymph (insect’s blood). The hemolymph contains waste and nutrients that are reabsorbed.

• Desert animals have an extra long Henle’s loop = higher reabsorption of water (very concentrated urine)

Homeostasis/Urine Production in Other Organisms

• Freshwater fish have osmotic water gain over their body’s surface. They must excrete large amounts of very diluted urine.

• Saltwater fish have osmotic water loss over their body’s surface. They gain water by drinking the salt water and secreting salt from their gills. They excrete small amounts of highly concentrated urine.

Kidney dialysis